Turbidity currents in Cachi Hydropower Reservoir, Costa Rica

The Cachi Reservoir is located in Costa Rica, east of San Jose. It has a volume
of 53 million qubic meters and a maximum depth of 68 m. The average annual
sediment inflow is 244 000 tons and the annualy average water inflow is
104 qubic meters/second. Density currents often form in the reservoir, caused
by a combination of temperature gradients and hight sediment concentration.

The bathymetry of the reservoir is shown in the figure below.

The colours show the depth, where blue is the largest depth, and red is the
smallest depth.

The SSIIM
model was used for the CFD calculations.
The lake was modelled with a strucured grid with hexahedral cells.
The grid is seen from above in the figure below:

The velocities in the reservoir for a steady situation witout density
variations is shown below. The inflow water velocity is 300 qubic meters/second.

The colours show the horizontal velocity, where blue is the smallest velocity
and red is the highest veloctiy

The sediment concentration near the bed for a steady situation
witout density variations is shown below. The sediments have an average diameter
of 0.04 mm.

The colours show the sediment concentration near the bed, where blue is the
smallest concentration and and red is the highest concentration

The turbidity currents in the reservoir was calculated using an inflow sediment
concentration of 37 000 ppm, and taking into account the effect of density
differences on the water flow field. A time-dependent calculation was done,
with a time step of 10 seconds. After 2900 iterations (8 hours), the velocity in
the streamwise direction was high close to the bed and a return flow was
seen close to the water surface. This is shown on the figures below. Note the
main flow direction is from left to right.

Longitudinal profile along centerline of the reservoir (j=6) showing
velocities